Whatever is supposed to be holding the other side isn't. You'll need to get a wrench on it.

 

That’s my issue is I can’t get a wrench because the bolt hand from inside the frame. You install it by feeding up the bolt and a spacer and then pulling it down with a wire

 

try using a impact wrench..

 

Worst case, you can pull the bumper off and get to the bolts that way. It takes a bit of time but isn't really hard to do if you have the correct size wrench.

If you live in the salt belt, be sure to keep that hitch covered in some kind of rust preventative material. Mine looked like the titanic after a year. Their coating just plain sucks.

 

Yep, pulling the bumper off can help. Here are some pics from mine to show what you are in for:

The rear plastic just snaps in. Start by pulling all the clips inside the fender wells. The small clips are deceiving. You need to push the center pin inwards to release. There also clips all along the bottom edge.

With the plastic out of the way, remove the bolts and nuts holding the steel bumper.

Now you can use a long wrench to reach down inside the frame rail.

 

Should tighten up easily if you spin it fast enough (with the lock washer on it), impact, electric drill etc. Try prying/ pulling down on the nut while trying to tighten it.

 

You could also try putting a star washer between the bolt and the frame.

 

I had the same concern before I installed the Ecohitch on my Crosstrek. If the nut is spun fast enough the star washer under the head of the bolt will lock the head of the bolt against the frame rail and you can torque the nut. You should have enough threads left on the bolts that you can put two nuts on the bottom of the bolt threads. Jam those two nuts together to create another bolt head that will keep the bolt from spinning as you tighten the washer and nut against the frame rail. Remove the two jam nuts from the bolt and use as needed to prevent any other bolts from spinning. After I tightened the four nuts that secure hitch on I ran a fiber lock nut up against the torqued nuts just as added protection from any of the bolts coming loose.